Small changes to Import and Reduce repo documentation

What does this MR do and why?

Some things changes have been observed when trying to follow along with the documentation related to reducing your project repository sizes.

Below is a list of the changes:

• doc/user/project/settings/import_export.md - Clarify that the process is not relevant to GitLab.com and you would need to go to the Admin Area before attempting to modify these settings. You cannot modify it in the Group container/ project settings.
• doc/user/project/repository/reducing_the_repo_size_using_git.md, Step 4, remove the --mirror argument as it causes errors when running step 10. The error users will receive is --mirror can't be combined with refspecs.
• doc/user/project/repository/reducing_the_repo_size_using_git.md, Steps 7 and 9 - Corrected the directory as the filter-repo directory no longer sits in the .git folder. At least when I tested on my MacBook.

Screenshots or screen recordings

None

How to set up and validate locally

Can clone this project https://gitlab.com/gitlab-silver/cmarais/ticket_work/356348_reduce-repo-size.git and then follow the steps provided in the documentation.

MR acceptance checklist

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